Entrained bed coal gasifiers produce mixtures of hot, raw fuel gas and molten slag. The molten slag is composed primarily of inorganic, mineral-like materials that were embedded in the coal and have a melting point range of about 1900.degree. to 3000.degree. F. This molten slag is unlike the tarry matter or char produced by gasifiers that operate at lower temperatures than the entrained bed gasifiers. The tarry matter and char are primarily composed of organic materials which are converted to gas in an entrained bed gasifier system.
In the gasification process, most of the molten slag is separated from the hot, raw fuel gas within the gasifier by letting gravity pull the slag into a large voume of water below the gasification zone. However, due to the high operating temperature of the entrained bed gasifier, some molten slag becomes suspended within the hot gas as slagging ash. Hereinafter, the slagging ash may be referred to as slag, slagging particles, molten ash, ash particles, etc, and it is understood that all such terms are equivalents and refer to the same sticky particles suspended in molten form in a gas.
Because gas exit temperatures range between 2200.degree. F. and 3000.degree. F., depending on the properties of the coal being gasified, some fine molten slagging ash remains suspended within the hot raw gas which leaves the gasifier. The ash remaining in the gas is sticky and adheres to almost any cool surface it contacts and solidifies thereon. This adherence may cause various problems, including a build-up of the ash on the sides of vessels and other downstream equipment, such as synthesis gas (or syngas) coolers, that follow the entrained bed gasifier. If the gas is cooled below the melting point of the slagging ash, the slagging ash particles solidify to fly ash, which is not sticky and thus does not cause such build-up problems.
One method known in the art for cooling the gas and thereby removing the sticky slagging ash from the gas, is to inject water into the gaseous stream. This reduces the temperature of the gas to within the range of 600.degree. to 1000.degree. F. With this direct quench method, substantially all of the slag is converted to fly ash and is then washed away as a slurry of the water and fly ash without the aforesaid build-up or fouling problems.
A disadvantage of the direct quench method is that most of the high quality sensible heat of the gas is converted to low grade heat and is thus wasted. It is preferable to conserve the sensible heat of the gas, e.g. by utilizing it for electric power generation through the formation of steam in a syngas cooler, typically located downstream of the gasifier. To preserve as much sensible heat as possible, the gas may be cooled to a temperature just below the slagging ash softening point. However, by using the direct quench method, the temperature is lowered to 1000.degree. F. or less, and thus the high quality sensible heat is converted to low quality heat and/or lost.
Another known method for removing the molten ash from a hot gas without the loss of most high quality sensible heat is the recycle gas quench system in which the hot gas is cooled to a temperature just below the ash softening point. In this system cool, scrubbed gas, instead of water, is injected into an entrained bed gasifier product. The heat absorbed by this scrubbed gas is recovered and the gas is recycled. However, not all of the ash is removed by the latter technique, with the fly ash remaining in the gas still being capable of causing build-up, or fouling problems within the syngas coolers. To remove the remaining ash, a second step employing the above-mentioned direct quench method must be performed. Such a second step entails the same problems as previously mentioned. Additionally, this recycled gas quench method requires costly equipment to facilitate the expansion of the scrubbed gas. Further, its efficiency is low since the scrubbed gas must be recompressed for repeated use.
A third method for overcoming the build-up problems caused by slagging ash is to pass the hot gas through a tall radiant heat exchanger in an attempt to cool the gas sufficiently to solidify the ash. Subsequently, the cooled gas is passed through a convective heat exchanger. A disadvantage of this technique is that a large drop in temperature is necessary to remove all the ash. Thus, ash will deposit on the convective heat exchanger unless the radiant heat exchanger is extremely large. Such a large exchanger involves considerably more expense, and is thus undesirable. Additionally, this method, like the recycled gas quench method, must be supplemented by an inefficient water quench process, and thus has the additional problems attendant thereto.
Other efforts at removing impurities from fluid streams have included the use of granular bed filtering devices. In these devices, louvers or screens are placed across the gas inlet and outlet opening to maintain the granular bed in place. In all such devices, problems tend to arise due to the agglomeration of the impurities on the louvers or screens placed across the gas inlet opening that retain the bed.
All of the prior art processes discussed above have inherent disadvantages and inefficiencies in the removal of slagging ash from a hot gas. Thus, the removal of slagging ash from a hot gas without build-up problems caused by the ash has not been realized to date.